Je to jednoduche, staci pridat jeden klic do registru, viz: http://stackoverflow.com/questions/2537076/visual-studio-winform-designer-shows-design-errors-as-text-rather-than-html .
Stava se to u IE8 i IE9.
Pokud se vam to nechce editovat, muzete pouzit muj export z registru:
http://dl.dropbox.com/u/18192161/RenderHtmlWithoutSource.reg